The present invention concerns a mutation responsible for autosomal prelingual non-syndromic deafness and a method for the detection of this hereditary sensory defect for homozygous and heterozygous individuals. The invention concerns more particularly a specific deletion of at least one nucleotide in the connexin 26 (Cx 26) gene and especially in a guanosine rich region, notably between the nucleotides 27 and 32. The invention is also directed to the use of polynucleotide, or fragments thereof, for example as tools useful for the in vitro detection of a mutation of a gene belonging to the Cx26 gene family.
Profound or severe prelingual deafness affects one child in a thousand in developed countries (Morton N E. Genetic epidemiology of hearing impairment. In Genetics of hearing impairment. (The New York Acad Sci, New York 1991; 630:16-31). It is a major handicap as it impedes language acquisition.
According to studies performed in a U.S. population of children with non-syndromic (isolated) prelingual deafness and in whom an obvious environmental cause has been excluded, it is estimated that up to two-thirds of the cases have a genetic basis (Marazita M L, Ploughman L M, Rawlings B, Remington E, Arnos K S, Nance W E. Genetic epidemiological studies of early-onset deafness in the U.S. school-age population. Am J Med Genet 1993; 46:486-91). These forms are mainly sensorineural and are almost exclusively monogenic. The major mode of inheritance is autosomal recessive (DFNB), involving 72% to 85% of cases, this fraction increasing to 90% when only profound deafness is taken into account.
Autosomal recessive prelingual deafness is known to be genetically highly heterogeneous. Estimates of the number of DFNB loci vary from thirty to one hundred (Petit C. Autosomal recessive non-syndromal hearing loss. In Genetics and Hearing Impairment. Martini A, Read A P, Stephens D, eds (Whurr, London) 1996; 197-212), for a review), of which fourteen have so far been mapped to the human chromosomes (Petit C. Genes responsible for human hereditary deafness: symphony of a thousand. Nature Genet 1996; 14:385-91) for review, (Verhoeven K, Van Camp G, Govaerts P J, et al. A gene for autosomal dominant non-syndromic hearing loss (DFNA12) maps to chromosome 11q22-24. Am J Hum Genet 1997; 60:1168-74 and Campbell D A, McHale D P, Brown K A, et al. A new locus for non-syndromal autosomal recessive sensorineural hearing loss (DFNB16) maps to human chromosome 15q21-q22. J Med Genet 1997; in press).
A majority of the families attending genetic counseling clinics consist of normal hearing parents with a single deaf child who wish to know the risk of recurrence of the defect. In most cases, given the major role of environmental causes of prelingual deafness, it is not usually possible even to recognize whether the hearing loss is of genetic origin. Genetic counseling in such families would be greatly improved by an ability to detect DFNB mutations. In this respect, the high genetic heterogeneity of the condition represents a major obstacle.
After the initial identification of the DFNB1 locus on 13q11 in a large consanguineous Tunisian family (Guilford P, Ben Arab S, Blanchard S, et al. A non-syndromic form of neurosensory, recessive deafness maps to the pericentromeric region of chromosome 13q. Nature Genet 1994; 6:24-8), two studies performed on New Zealand/Australian families (Maw M A, Allen-Powell D R, Goodey R J, et al. The contribution of the DFNB1 locus to neurosensory deafness in a Caucasian population. Am J Hum Genet 1995; 57:629-35), and on Italian/Spanish families (Gasparini P, Estivill X, Volpini V, et al. Linkage of DFNB1 to non-syndromic neurosensory autosomal-recessive deafness in Mediterranean families. Eur J Hum Genet 1997; 5:83-8) suggested that this locus might be a major contributor to prelingual deafness in these populations, although individual lod scores obtained in these families were not significant owing to the small size of these families.
Recently, the Cx26 gene, which encodes a gap junction protein, connexin 26, has been shown to underlie DFNB1 deafness. Two different G->A substitutions resulting in premature stop codons in three DFNB1 linked consanguineous Pakistani families have been reported (Kelsell D P, Dunlop J, Stevens H P, et al. Connexin 26 mutations in hereditary non-syndromic sensorineural deafness. Nature 1997; 387:80-3). These two substitutions were identified, respectively, at codon 77 and at codon 24. This result has offered the opportunity directly to assess this hypothesis.
The difficulties encountered in genetic counseling for prelingual non-syndromic deafness due to the inability to distinguish genetic and non-genetic deafness in the families presenting a single deaf child was one of the reasons that led the inventors to undertake a characterization of the spectrum and prevalence of mutations present in the Cx26 gene in 35 families from several parts of the world with autosomal recessive prelingual deafness.